题目

Zoo elephants don’t live as long as those in the wild, according to a study sure to cause debate about keeping the giant animals on display. Researchers compared the life spans of elephants in European zoos with those living in Amboseli National Park in Kenya and others working on a timber （木材） enterprise in Burma. Animals in the wild or in natural working conditions had life spans twice that or more of their relatives in zoos. Animal care activists have urged in recent years to discourage keeping elephants in zoos, largely because of the lack of space and small numbers of animals th 根据原文默写、填空。 君问归期未有期，________。（《夜雨寄北》）